Dark Energy

Dark energy is the name scientists give to the unknown cause of the universe’s accelerating expansion, making distant galaxies grow farther apart increasingly quickly.

Imagine dots drawn on a stretchy ribbon. If the ribbon begins stretching faster and faster, the distant dots move apart more quickly—not because the dots themselves are growing, but because the space between them is expanding.

Astronomers discovered that the expansion of the universe is accelerating by observing distant exploding stars and measuring how cosmic expansion has changed over time. Dark energy is the name given to whatever is causing this acceleration, but scientists do not yet know its true nature. One leading possibility is that empty space itself contains a constant form of energy, sometimes described by Einstein’s cosmological constant. Dark energy is not the same as dark matter: dark matter adds gravitational attraction and helps galaxies form and remain together, while dark energy acts across immense distances and appears to accelerate the expansion of space. Understanding it remains one of the greatest open questions in cosmology.